This invention relates to timing recovery and frame synchronization in communications systems. The invention is applicable to any communications system having known sync (synchronization) sequences or words, and is especially applicable to, and is described below in the context of, a TDMA (time division multiple access) cellular communications system compatible with EIA/TIA document IS-54-B: Cellular System Dual-Mode Mobile Station--Base Station Compatibility Standard (Rev. B). For convenience and brevity, such a system is referred to below simply as an IS-54 system. In such a system, data is communicated in time slots each comprising a sync word of 14 symbols followed by an information sequence.
It is well known that it is necessary in communications systems to recover the timing and synchronize to the time division multiplex (TDM) frames of a received digital data communications signal, so that samples of the signal are obtained at optimum times for further processing to recover the communicated data. It is also well known that timing recovery, frame synchronization, and the necessary processing of the samples are made more difficult by a low signal-to-noise ratio (SNR), and that a low SNR can often be present in cellular communications systems.
Denoting the symbol spacing of the received signal, and hence the period between successive samples, as T, and denoting the sampling delay, i.e. the period between the optimal and actual sampling times of the received signal, as .tau., then frame synchronization serves to ensure that the sampling delay .tau. is within one symbol spacing, i.e. .tau. is within the interval from -T/2 to T/2, and timing recovery serves to reduce the sampling delay .tau. to substantially zero. In practice, a sampling delay control signal can be used to adjust the actual sampling times or, equivalently, to control an interpolator to which the actual samples are supplied to obtain interpolated samples at the optimal sampling times, whereby frame synchronization and timing recovery is achieved.
An object of this invention is to provide improved frame synchronization and timing recovery in a communications system.